What it is

A sales tax audit is a formal review by a state’s department of revenue (DOR) — in California, the CDTFA; in Texas, the Comptroller — of whether your business collected, reported, and remitted sales and use tax correctly. The audit usually opens with an engagement letter naming your assigned auditor and specifying the look-back period, which is most commonly three or four years from a return’s due date or filing date, whichever is later. Most states can reach back three years; the window can extend if returns were never filed or if fraud is alleged.

The audit sits squarely in the remediation half of the compliance lifecycle. It tests the work the rest of the lifecycle was supposed to get right: did you have nexus and register, did you apply the correct taxability, and can you document every exempt sale? Where the records fall short, the state issues an assessment — the additional tax it believes you owe, plus penalties and interest.

Why it matters to a multi-state seller

The more states you sell into, the larger your audit surface. Each registration is a relationship with a DOR that can examine you, and the things that draw an auditor’s attention are common across them: discrepancies between your sales tax returns and your income tax filings, late or missing returns, sudden swings in reported sales, large volumes of claimed exempt sales, or being swept into an industry-wide audit initiative. Economic nexus has widened the net further — states now audit remote sellers they would never have reached before Wayfair.

The financial stakes are asymmetric. You collected the tax (or were supposed to) from your customers, but once an audit finds under-collection, the liability is yours — you generally cannot go back and bill past customers for tax you failed to charge. Assessments routinely run into five and six figures because under-collection compounds across thousands of transactions and several years.

Worked example

Say a wholesale-and-retail seller registered in California gets an audit notice covering a three-year look-back. The CDTFA auditor focuses on three areas:

• Exemption certificates. The seller made $400,000 in “resale” sales but can produce valid resale certificates for only $310,000 of them. The $90,000 with missing or incomplete certificates is reclassified as taxable, because a certificate missing a required field — name, address, tax ID, reason, signature, or date — invalidates the exemption.
• Taxability. A product line the seller treated as non-taxable is found taxable in the state, adding to the base.
• Nexus and reporting. The auditor confirms returns were filed but spots a quarter where reported sales don’t match the seller’s own records.

The auditor totals the under-collected tax into an assessment, then adds penalty and interest. The seller’s exposure now hinges on documentation it should have collected at the time of sale.

State-level nuance

Audits are run by each state under its own statute, but the look-back window and authority follow a recognizable pattern:

The constant across states is what auditors test: nexus and registration, the taxability of what you sold, and the validity of every exemption certificate behind an untaxed sale. Use tax — tax owed on your own taxable purchases where no sales tax was charged — is frequently reviewed alongside sales tax.

How this connects to staying compliant

An audit rewards good records and punishes gaps. The defenses that hold up are built long before the notice arrives: collect and validate exemption certificates at the time of sale, keep your sales tax returns reconciled to your income tax filings, and file every period on time. When a notice does arrive, how you respond materially affects the outcome — which is why representation matters. See sales tax audit defense.

Sources: Thomson Reuters — What triggers a sales tax audit and how do you reduce the risks?: https://tax.thomsonreuters.com/blog/what-triggers-a-sales-tax-audit-and-how-do-you-reduce-the-risks/ Avalara — What is a sales tax audit and what happens if I get audited?: https://www.avalara.com/blog/en/north-america/2024/02/what-is-a-sales-tax-audit-what-happens-if-i-get-audited.html California CDTFA — Publication 76, Audits: https://cdtfa.ca.gov/formspubs/pub76.pdf